Sorry about that, the .app extension is for Mac. How about building it following this guide? I'm currently at work so can't test it myself at the moment.

Building
Run download-nwjs.sh to download the latest nw.js binaries
Download the TiddlyWiki5 repo from https://github.com/Jermolene/TiddlyWiki5 to a sibling directory to the TiddlyDesktop repo called "TiddlyWiki5"
Run bld.sh
Execute output/linux32/nw or output/linux64/nw